Radioactive halogen monitoring system

ABSTRACT

A system of two or more filter-detectors for detecting radioactive halogens such as iodine in a gas stream also containing other radioactive substances such as radioactive noble gases.

United States Patent [1 1 [111 3,769,505 Lee et al. Oct. 30, 1973 [54] RADIOACTIVE HALOGEN MONITORING 2,951,156 8/1960 Miller 250/435 MR SYSTEM 2,945,127 7/1960 Hanson 250/435 MR Inventors: John S. Lee, Morgan Hill; William R. Alves, Mt. View, both of Calif.

Assignee: General Electric Company, San

Jose, Calif.

Filed: Mar. 15, 1972 Appl. N0.: 234,922

U.S. Cl ..250/394, 250/430, 250/395 Int. Cl. G0ln 21/26 Fleld of Search 250/435 MR, 83.6 FT

References Cited UNlTED STATES PATENTS Tone 250/435 MR FOREIGN PATENTS OR APPLICATIONS 837,735 3/1970 Canada 250/435 Primary Examiner-James W. Lawrence Assistant Examiner-Harold A. Dixon Attorneylvor J. James et al.

57 ABSTRACT A system of two or more filter-detectors for detecting radioactive halogens such as iodine in a gas stream also containing other radioactive substances such as radioactive noble gases.

13 Claims, 6 Drawing Figures i RATE f METER I E ;///2/ mm) I e 22/2) I/ FLEET: q/arzl GAS l l 23/2) /8 2570"I l I TIMER RADIOACTI'VITY PAIENIED URI 3 0 I973 FILTER IOOII) FILTER FILTER IOO(3) SHEET 30F 3 TIME (UNITS) Fig. 6

RADIOACTIVE HALOGEN MONITORING SYSTEM BACKGROUND The invention relates to the detection of radioactive substances and particularly to the monitoring of radioactive halogens such as iodine in the presence of radioactive noble gases such as fission product gases. For example, the process of fission in the operation of a nuclear power plant results in the formation of fission products among which are radioactive iodine and radioactive noble gases such as krypton and xenon. These fission products are formed in the fuel and normally are retained in the sealed fuel rods. In practice small amounts of the fission gases and volatile fission products escape into the coolant. In addition small quantities of radioactive gases are formed by activation in the coolant. Because of the relatively short half-lives of most of these radioactive substances and the small total amount thereof, they safely may be released to the atmosphere after suitable treatment and delay. Such a nuclear reactor off-gas treat-ment system is described, for example, by H. J. Schroeder et al in Off-gas Facility at the Gundremmingen Nuclear Power Plant, Kerntechnick 13 (1971) No. 5, page 205.

In addition to monitoring the gross radioactivity of the gases, vapors and particulates of the off-gas stream it is desirable to separately monitor the radioactivity of particular ones of these substances, for example, iodine. The monitoring of iodine has been found to be very difficult because it constitutes but a small fraction of the off-gas stream and its radioactivity is masked by the greater radioactivity of the noble gases. However, iodine can be captured and accumulated in an activated charcoal filter while the accompanying noble gases, though adsorbed and delayed in their passage through such a filter, are not accumulated therein. Thus a known sample amount of the off-gas stream can be passed through a charcoal filter for a given period of time. The filter then can be flushed (with clean air for example) to remove any remaining noble gases and then after a time sufficient to let solid radioactive decay products of the gases (for instance cesium and rubidum) to decay to a sufficiently low level (their half lives being much shorter than most of the iodine) the radioactivity of the accumulated iodine can be measured. By relating this measurement to the sample amount and time the average contribution of the iodine over the sample period to the gross radioactivity of the off-gas stream can be determined. This approach to iodine monitoring is employed, for example, in the off-gas system of the aforementioned Schroeder et al article. A serious disadvantage of such an approach is that the sampling period is undesirably long, for example, in the order of several days. It is an object of the invention to greatly reduce the sampling time. Another object is to eliminate the need of periodic removal and hapdling of the filter element.

SUMMARY These and other objects are achieved by a system including two (or more) iodine collecting filters and radiation detectors. A sample stream of the gas to be monitored (such as the reactor off-gas) is pumped through a first one of the filters while a second filter is flushed with a purge gas and the radiation from the second filter is measured. After a relatively short sampling pe riod, the sample stream is diverted from the first filter to the second filter and the first filter is flushed and measured. At the end of the second sampling period, the sample stream is again diverted to the first filter etc. With this cyclic technique the monitoring system response is as though it were measuring the accumulation of iodine in a filter where the radioactive noble gases are not present.

DRAWING The invention is described more specifically hereinafter with reference to the accompanying drawing wherein:

FIG. 1 is a schematic diagram of an iodine monitoring system in accordance with-the invention;

FIG. 2 is a timing diagram of the operation of the system of FIG. 1;

FIG. 3 illustrates the inclusion of a differential circuit in the system;

FIG. 4 illustrates the inclusion of a sample gas heater in the system;

FIG. 5 is a schematic diagram of a system employing three filters; and

FIG. 6 is a timing diagram of the operation of the system of FIG. 5.

DESCRIPTION As illustrated, by way of example, in FIG. 1, the system of the invention comprises a pair of well-known activated charcoal filters 10(1) and 10(2), a pair of twoway valves 11(1) and 11(2), a pump 12, a pair of wellknown radiation detectors 13(1) and 13(2), for example, of the sodium iodide crystal type, a switch 14 for connecting the detectors 13(1) and 13(2) alternately to a pulse rate meter 16 (including an indicator l7) and a timer 18 for cyclically actuating the valves 11(1), 11(2) and the switch 14. (Radiation detectors and pulse rate meters are described for example by WJ. Price in Nuclear Radiation detection, 2nd edition,

McGraw-I-Iill.)

Operation of the system of FIG. 1 is illustrated by the timing diagram of FIG. 2 which shows the radioactivity, as detected at the filters 10(1) and 10(2) plotted against arbitrary time units. Initially, the valve 11(1) is set so that sample gas from a sample gas input conduit 19 is fedthrough filter 10(1) wherein the iodine contained in the sample gas is trapped and the accompanying noble gas exhausted from the filter 10(1) through an output conduit 21 by pump 12.

Meanwhile, the valve 11(2) is set such that the filter 10(2) is being flushed with a purge gas (such as clean air, steam or the like) from a purge gas input conduit 22(2) and the switch 14 is set so as to connect detector 13(2) to the rate meter 16.

At the end of the fourth time unit, the valve 11(1) is actuated by the timer 18, via a link 23(1), to shut off the sample gas and to direct a purge gas from a purge gas input conduit 22(1) through the filter 10(1). At the end of the fifth time unit, the valve 11(2) is actuated by the timer 18, via a link 23(2) to feed sample gas through the filter 10(2); simultaneously, the timer 18, via a link 24, throws switch 14 to its alternate position to thereby connect the detector 13( 1) to rate meter 16. At the end of the ninth time unit, the valve 11(2) is actuated to flush the filter 10(2), and one cycle of operation is completed at the end of the tenth time unit when the valve 11(1) is again actuated to direct sample gas through filter (1) and the switch 14 is thrown to again connect detector 13(2) to rate meter 16.

Thus during each cycle of system operation each filter collects for four time units, is flushed for six time units and is monitored during the last five time units of each flushing cycle. This provides one time unit of flushing cycle overlap which serves to substantially purge the noble gases from a filter before the adjacent detector is connected to the rate meter.

As mentioned above, the time units shown in FIG. 2 are arbitrary; however, it is contemplated that the collecting periods (of four time units) may be a fraction of time required by prior systems, for example, in the order of 10-30 seconds. Thus the present system provides a more prompt indication of the amount of iodine in the sample gas stream, especially as compared to the many hours, or even days, of collecting time of the known prior iodine monitoring systems.

As shown in FIG. 2, the radioactivity detected at each filter increases with time (to the right) as more and more iodine is collected in the filters. However, if the amount of radioactive iodine in the sample gas remains constant, the measured radioactivity in the filters will reach a constant value at an equilibrium point where the rate of decrease of radioactivity due to decay equals the rate of radioactivity build-up due to the newly collected iodine. Any change in the amount of iodine in the sample gas will shift the equilibrium point and cause a corresponding change in the reading on the indicator 17.

For'some applications it may be desirable to provide an indication of the rate of change of radioactivity from monitoring period to monitoring period. This can be provided by a well-known differentiating circuit which, in effect, stores and divides the measurement of the previous monitoring period into the measurement of the current monitoring period. Such a differentiating circuit can be connected to replace the rate meter 16 or it can be connected in parallel therewith as shown in FIG. 3 which illustrates a differentiating circuit 25, with an indicator 26, connected to the switch 14 in parallel with rate meter 16. (Suitable differentiating circuits are shown by WJ. Price in the above mentioned publication.)

it has been found that the effectiveness of the iodine monitoring system of the invention can be increased by heating the sample gas before it is fed to the charcoal fiters. Heating of the sample gas is found to reduce the amount of noble gases residing in the filters and consequently their radioactive decay products, such as eesiurn and rubidium, with which they are in radioactive equilibrium, while not appreciably reducing the ability of the filters to collect and retain the iodine, thus reducing necessary cesium and rubidium decay times before measurement of iodine. The temperature of the sample gas can be raised by installing a heater 27 in the sample gas input conduit 19 as illustrated in FIG. 4. Good results may be obtained by heating the sample gas to a temperature in the range of l50-250F.

While the system of the invention is described herein in connection with monitoring of reactor off-gases, the system is equally useful for other applications such as monitoring the iodine or other halogens in the atmosphere of reactor, heat exchanger or turbine containments or in reprocessing plants or for any other application requiring the detection of radioactive iodine or other halogens in the presence of radioactive noble gases or other substances which effectively can be flushed from the filters.

The concept of the invention exemplified in FIGS. 1 and 2 can be extended through the use of additional filter elements and associated valves and detectors. A system using three filters i00(l)-100(3), with associated valves (1)-1l0(3), detectors l30(l)l30(3) and a three-position stepping switch is shown in FIG. 5. Operation of the system of FIG. 5 is illustrated by the timing diagram of MG. 6 wherein the collecting (C) and monitoring (M) periods are each of three time units duration while the flushing periods (F) are of six time units duration. The additional filter allows a longer flushing and decay time before measurement for more complete purging of the noble gases from the filters.

What is claimed is:

l. A system for detecting radioactive halogen in a stream of sample gas also containing radioactive noble gas, comprising: first and second charcoal filters each of said filters having an inlet and an outlet; means for discharging gas from the outlets of said filters; a source of said sample gas; a source of purge gas; a first valve having an outlet connected to said inlet of said first filter, an inlet connected to said source of sample gas and an alternate inlet connected to said source of purge gas; a second valve having an outlet connected to said inlet of said second filter, an inlet connected to said source of sample gas and an alternate inlet connnected to said source of purge gas; means for actuating said first and second valves; a first radiation detector located adja cent said first filter; and a second radiation detector located adjacent said second filter.

2. The system of claim 1 including a rate meter for counting and indicating the radiation events detected by said detectors; and a switch having a pole and first and second terminals, said pole being connected to said I rate meter, said first terminal being connected to said first detector and said second terminal being connected to said second detector.

3. The system of claim 2 wherein said means for actuating includes timing means having a first actuating link connected to said first valve, a second actuating link connected to said second valve, and a third actuating link connected to said switch.

4. The system of claim 1 including means for increasing the temperature of said sample gas including a heater connected between said source of sample gas and said first and second valves.

5. The system of claim 1 including a differentiating circuit connected to said detectors for indicating the rate of change of radiation detected by said detectors.

6. A system for detecting radioactive halogen in a sample stream also containing other radioactive substances, comprising: a plurality of filter elements having the capability of collecting and retaining said halogen and allowing said other substances to be substantially flushed therefrom by a purge stream; means for directing said sample stream in turn through successively different ones of said elements during successive collecting periods; means for directing said purge stream in turn through said successively different ones of said elements during successive flushing periods following said collecting periods; and means for detecting radiation in turn from said successively different ones of said elements during successive monitoring periods starting after the beginning of the flushing period and ending at least at the beginning of a next successive collecting period of each element.

7. The system of claim 6 including means for heating said sample stream to a temperature of l50-250F. before passage into said filter elements.

8. The system of claim 6 wherein said filter elements contain activated charcoal.

9. The system of claim 6 including means for counting and registering the radiation events detected in said filter elements.

10. The system of claim 6 including means for indicating the rate of change of radiation from each of said filter elements.

11. A method of detecting radioactive halogen in a gas stream also containing radioactive noble gas comprising the steps of:

a. providing a plurality of filter elements having the capability of collecting and retaining said halogen and allowing said noble gas to be flushed therefrom;

b. collecting said halogen periodically and in turn in each of said filter elements by directing said gas stream through successively different ones of said filter elements during successive collecting periods;

c. flushing noble gas periodically and in turn from each of said filter elements by directing a stream of purge gas through successively different ones of said filter elements during successive flushing periods following said collecting periods; and

d. detecting radiation in turn from successively different ones of said filter elements after the beginning of the flushing period and before the beginning of a next successive collecting period of each filter element.

12. The method of claim 11 including the step of heating said gas stream to a temperature of -250F before directing said gas stream through said filter elements.

13. The method of claim 11 wherein said filter elements contain activated charcoal. 

2. The system of claim 1 including a rate meter for counting and indicating the radiation events detected by said detectors; and a switch having a pole and first and second terminals, said pole being connected to said rate meter, said first terminal being connected to said first detector and said second terminal being connected to said second detector.
 3. The system of claim 2 wherein said means for actuating includes timing means having a first actuating link connected to said first valve, a second actuating link connected to said second valve, and a third actuating link connected to said switch.
 4. The system of claim 1 including means for increasing the temperature of said sample gas including a heater connected between said source of sample gas and said first and second valves.
 5. The system of claim 1 including a differentiating circuit connected to said detectors for indicating the rate of change of radiation detected by said detectors.
 6. A system for detecting radioactive halogen in a sample stream also containing other radioactive substances, comprising: a plurality of filter elements having the capability of collecting and retaining said halogen and allowing said other substances to be substantially flushed therefrom by a purge stream; means for directing said sample stream in turn through successively different ones of said elements during successive collecting periods; means for directing said purge stream in turn through said successively different ones of said elements during successive flushing periods following said collecting periods; and means for detecting radiation in turn from said successively different ones of said elements during successive monitoring periods starting after the beginning of the flushing period and ending at least at the beginning of a next successive collecting period of each element.
 7. The system of claim 6 including means for heating said sample stream to a temperature of 150*-250*F. before passage into said filter elements.
 8. The system of claim 6 wherein said filter elements contain activated charcoal.
 9. The system of claim 6 including means for counting and registering the radiation events detected in said filter elements.
 10. The system of claim 6 including means for indicating the rate of change of radiation from each of said filter elements.
 11. A method of detecting radioactive halogen in a gas stream also containing radioactive noble gas comprising the steps of: a. providing a plurality of filter elements having the capability of collecting and retaining said halogen and allowing said noble gas to be flushed therefrom; b. collecting said halogen periodically and in turn in each of said filter elements by directing said gas stream through successively different ones of said filter elements during successive collecting periods; c. flushing noble gas periodically and in turn from each of said filter elements by directing a stream of purge gas through successively different ones of said filter elements during successive flushing periods following said collecting periods; and d. detecting radiation in turn from successively different ones of said filter elements after the beginning of the flushing period and before the beginning of a next successive collecting period of each filter element.
 12. The method of claim 11 including the step of heating said gas stream to a temperature of 150*-250*F before directing said gas stream through said filter elements.
 13. The method of claim 11 wherein said filter elements contain activated charcoal. 